"A Mirror or a 2-Way Glass? How can you tell when you are in a room, restroom, motel etc. with a mirror or a 2-way glass...try this to test mirrors in fitting rooms and restrooms..."
This is a common urban legend that has left the confines of email chain letters and found it's way into Pinterest. To quote the website this image was pinned from "Just conduct this simple test: Place the tip of your fingernail against the reflective surface and if there is a GAP between your fingernail and the image of the nail, then it is GENUINE mirror. "
It seems like a simple enough test that we can all do once we're in a fitting room, to see if there are cameras hidden behind the mirrors. But unfortunately, it doesn't work. Simply holding your finger up to a mirror and checking for gaps isn't going to let on what kind of mirror you're looking at. A lot of different factors can go into if there will be a gap or not when you press your finger to the mirror. Lower quality mirrors won't always give you a gap, even though they are genuine mirrors.
There are a couple of tricks you can try when trying to figure out if you're being spied on, the easiest and most accurate is to simply cup your eye with your hands and look into the mirror - this will allow you to limit the light source on your end, allowing you to be able to see the other side. Another hint is if the mirror is hanging on a wall instead of being set into a wall. (source)
Another thing to take in account is that the laws regarding cameras in fitting rooms varies greatly depending on the state or country you live in. Some allow them only in the walkway, some in the rooms, and some not past the entrance of the fitting rooms. The best thing you can do is learn your local laws before you start trying to poke every mirror you see.